This is an alternate take on Season 9 Episodes 24 and 25 "The ending of the End".
Here Cozy Glow feels remorseful for her past actions and wants to redeem herself and make amends with everyone she has hurt. Therefore she...

betrays Lord Tirek and Queen Chrysalis by joining Mane 6 to fight them in "the Ultimate Battle for the fate of Equestria". But her betrayal won't stop the remaining villains nor make them surrender...

They will find another way to defeat Twilight and take over Equestria